"It has the potential to infect about 30% of Windows systems online, a potential 300 to 350 million PCs," says Don Jackson, director of threat intelligence in the counter threat unit at SecureWorks. The worm, first identified in November and suspected to have originated in the Ukraine, is quickly ramping up, and while Downadup today is not malicious in the sense of destroying files — its main trick is to block users from accessing antivirus sites to obtain updates to protect against it — the worm is capable of downloading second-stage code for darker purposes. Many experts anticipate that could occur soon.

What that darker purpose might be is a source of speculation, but Jackson theorizes that it will may well end up being "rogue antivirus malware" that demands the user buy it to eliminate the worm. "It's basically extortion," he says.

Like SecureWorks, IBM notes that it's the second stage payload of the Downadup worm that is a source of concern. "Right now it's not destroying or stealing,--it's just hanging out," comments Tom Cross, X-Force researcher in the IBM ISS division. "It's building its network of hosts."

While no one knows exactly what stage two payload will bring, one reason for the worm's somewhat slow but steady progress is its use of Windows "AutoRun" to copy itself through Windows file-sharing and USB tokens, Cross says.

"If it copies itself to a file share, and if the user clicks on a file, the user's computer will get infected," Cross says. "Even if the computer is patched, you can still get infected if you access one of the infected USB drives or file shares." Cross advises that AutoRun be disabled.

This is an additional means of the worm spreading beyond exploiting the Windows RPC flaw identified last October, for which a patch is available. The worm also has a password-cracker that is adept at cracking administrative accounts or other computers, though very strong passwords should make that much harder, Cross says.